Easily transportable devices with wireless telecommunications capabilities, such as mobile telephones, personal digital assistants, handheld computers, and similar devices, will be referred to herein as mobile devices. Some mobile devices communicate in a circuit switching mode, wherein a dedicated communication path may typically exist between two devices. For the duration of a call, all data exchanged between the two devices travels along the single path. An example of a telecommunications protocol that uses circuit switching is the Global System for Mobile Communications (GSM).
Some mobile devices also have the capability to communicate in a packet switching mode. In packet switching, a data stream is divided into packets that are given unique identifiers. The packets might then be transmitted from a source to a destination along different paths and might arrive at the destination at different times. Upon reaching the destination, the packets are reassembled into their original sequence based on the identifiers. An example of a telecommunications protocol that uses packet switching is the Session Initiation Protocol (SIP).
Communications that take place via circuit switching can be said to occur in the circuit switching domain and communications that take place via packet switching can be said to occur in the packet switching domain. Mobile devices that can communicate in only the circuit switching domain or only the packet switching domain can be referred to as single domain devices or single mode devices. Mobile devices that can communicate in both the circuit switching domain and the packet switching domain can be referred to as dual domain devices or dual mode devices. A communications connection in the circuit switched domain or in the packet switched domain can be referred to as a call or a session. These calls or sessions may carry voice or data.